Non-conceptive sex culture in Japanese macaques

We investigated the ontogeny
and causal mechanisms underlying another non-adaptive behavior, namely,
homosexual behavior in female Japanese macaques, and its relationship to a
purportedly adaptive behavior, namely, female-male mounting. [REPORT 1][REPORT 2]

We showed that the emergence of both conceptive and non-conceptive sexual behaviors in
female Japanese macaques can be traced back to adolescence, with a major developmental threshold occurring at
four years of age.[REPORT 1][REPORT 2]

However, we found marked
differences in the development of heterosexual and homosexual behaviors that I
explained in terms of aggression risk, social facilitation and sexual reward.[REPORT]

Our research on female homosexual behavior and inter-sexual mate competition in Japanese macaques has implications for sexual selection theory. [REPORT]

After testing a series of
hypotheses about the influence of female-biased sex ratios on females’ sexual
partner preferences, our results supported the “bisexual preference” hypothesis
and suggested that when a female targets another female as a mate, it is an
active choice of a female sexual partner over male alternatives.[REPORT]

We also found substantial intergroup differences and covariation in the frequency and
form of female-female mounting[VIDEO]and female-male mounting[VIDEO].

Our results supported the
view that these two forms of non-conceptive sexual behaviors were
developmentally and evolutionarily linked, and might be cultural practices arising
in groups when certain socio-demographic conditions are met.[REPORT]

Whereas male mounting posture should be optimal (i.e., precisely
coordinated and invariant) in order to achieve penile intromission during
heterosexual copulation, female mounting is less functionally constrained,
which allows for more flexible and arbitrary behavioral patterns.

We showed that the customary occurrence, high prevalence, and great
diversity of female-female and female-male mounts at Arashiyama may be the result of combined favorable socio-demographic
conditions, namely few resident males, most of them being old, sexually
under-motivated, and less aggressive and controlling than the average male
Japanese macaques living in the other study groups at Minoo and Jigokudani.

We argued that although genetic
explanations for such intraspecific variation cannot be rules out, arbitrary behavioral patterns such as
intergroup differences in female mounting postures in Japanese macaques could
be purely cultural, as any alternative explanation is difficult to imagine.

We providedthe first reportofmale homosexual consortships and mountswithin male-male dyads in a free-ranging all-male group of Japanese
macaques, at Minoo, central Japan. Male homosexual interactions
shared most of the behavioral components that have been reported to
characterize heterosexual and female homosexual consortships in this species(e.g., male-male solicitations, mounting
postures, body orientations, inter-mount activities, and third-party male
intrusions).

We argued that research on male homosexual behavior in all-male groups of primates
under natural settings, and even more interestingly, studies of male bisexual behavior
expressed in the dual socio-demographic context of all-male and mixed-sex
groups of primates may provide insights into the developmental
processes, causal mechanisms, adaptive significance, and phylogenetic pathways
that characterize male bisexuality in humans.